The present invention relates to a sintering method for cemented carbide for the purpose of eliminating the binder phase layer from its surface before applying coatings on said surface.
Coated cemented carbide inserts have now for many years been commercially available for chip forming machining of metals in the metal cutting industry. Such inserts are commonly made of a metal carbide, normally WC, generally with the addition of carbides of other metals such as Nb, Ti, Ta, etc. and a metallic binder phase of cobalt. By depositing onto said inserts a thin layer of a wear resistant material such as TiC, TiN, Al.sub.2 O.sub.3 etc., separately or in combination, it has been possible to increase the wear resistance toughness essentially maintained.
During sintering cemented carbide inserts often obtain a completely or partly covering binder phase layer generally &lt;1 .mu.m thick on their surface. This particularly applies to inserts with a binder phase enrichment in the surface below the coating, so called cobalt gradient, but also to inserts with an even distribution of binder phase. In the latter case, this layer forms on certain grades but not on others. The reason for this is not understood at present. However, the layer has a negative effect on the process when carrying out CVD- or PVD-deposition, which results in layers with inferior properties and insufficient adherence. The binder phase layer must therefore be removed before carrying out the deposition process.
It is possible to remove such a binder phase layer mechanically by blasting. The blasting method is, however, difficult to control. The difficulty resides in the inability to control consistently the blasting depth with the necessary accuracy, which leads to an increased scatter in the properties of the final product--the coated insert. It also results in damages to the hard constituent grain of the surface. However, in Swedish patent application 9202142-7 it is disclosed that blasting with fine particles gives an even removal of the binder phase layer without damaging the hard constituent grains.
Chemical or electrolytic methods could be used as alternatives for mechanical methods. U.S. Pat. No. 4,282,289 discloses a method of etching in a gaseous phase by using HCl in an initial phase of the coating process. In EP-A-337 696 there is proposed a wet chemical method of etching in nitric acid, hydrochloric acid, hydrofluoric acid, sulphuric acid and similar or electro-chemical methods. From JP 88-060279 it is known to use an alkaline solution, NaOH, and from JP 88-060280 to use an acid solution. JP 88-053269 discloses etching in nitric acid prior to diamond deposition. There is one drawback with these methods, namely, that they are incapable of only removing the cobalt layer. They also result in deep penetration, particularly in areas close to the edge. The etching medium not only removes cobalt from the surface but also penetrates areas between the hard constituent grains. As a result, an undesired porosity between layer and substrate is obtained at the same time as the cobalt layer may partly remain in other areas of the insert. U.S. Pat. No. 5,380,408 discloses an etching method according to which electrolytic etching is performed in a mixture of sulphuric acid and phosphoric acid. This method gives an even and complete removal of the binder phase layer without depth effect, i.e. reaching zero Co-content on the surface.
On the other hand it is in some cases not desirable to reach a zero Co-content on the surface from a coating adhesive point of view, but rather a Co surface content close to nominal content.
The above mentioned methods require additional production steps and are for that reason less attractive for production in a large scale. It would be desirable if sintering could be performed in such a way that no binder phase layer is formed or alternatively can be removed during cooling.